substitution for stem length

[Stefan Thomas]

Dear Lilypond-users,
I would like to make a varibable substitution for stems over two systems. 
Before I used (thanks to Wilbert Berendsen)
noFlag = \override Stem #'flag-style = #'no-flag

stemExtend = { \noFlag \autoBeamOff
\override TupletNumber #'transparent = ##t
\override TupletBracket #'bracket-visibility = ##f
\override Stem #'length = #16
\override Stem #'cross-staff = ##t
       
}
Unfortanetely the stem length of 16 isn't always good.
Now I would like to type something like " langhals #16 " c4
I tried it, wothout success, with:
Langhals = #(define-music-function (parser location padding) (number?)
       #{
         \once 
     \override Stem #'length = #$length
     
       #})